1. Field of the Invention
Example embodiments of the present invention relate to a semiconductor device and a method of manufacturing a semiconductor device. More particularly, example embodiments of the present invention relate to a semiconductor device including a transistor having a channel substantially perpendicular to a substrate, and a method of manufacturing the semiconductor device.
2. Description of the Related Art
In a semiconductor memory device, a transistor includes a source region providing charge carriers, for example, electrons or holes, a drain region draining the charge carriers from the source region, and a gate electrode adjusting a current flowing between the source and the drain regions. When the current is adjusted by a voltage applied to the gate electrode of the transistor, the transistor is referred to as a field effect transistor. Here, a channel of the transistor includes a passage through which the charge carriers pass from the source region to the drain region. The channel is provided between the source and drain regions. The transistor further includes a gate insulation layer formed between a substrate and the gate electrode for electrically insulating the gate electrode from the substrate.
As the degree of integration degree of the semiconductor memory device increases, the cross-sectional area of the gate electrode sharply decreases. As the cross-sectional area of the gate electrode decreases, problems such as a short channel effect may occur in the transistor. The short channel effect causes various problems such as an increase of a leakage current in the transistor, a decrease of a breakdown voltage of the transistor, a continuous increase of a current caused by a voltage applied to the drain region, etc.
The short channel effect is generally generated by a shortened length of the channel between the source and the drain regions. Recently, a transistor having a source region and a drain region formed perpendicular to a substrate has been suggested. The transistor includes a channel formed perpendicular to the substrate between the source and drain regions.
FIG. 1 is a perspective view illustrating a conventional semiconductor device and FIG. 2 is a cross-sectional view illustrating the conventional semiconductor device.
Referring to FIGS. 1 and 2, a transistor having perpendicular channels includes lower active patterns 12 protruding from a substrate 10, upper active patterns 14 having pillar structures formed on the lower active patterns 12, gate insulation layer patterns (not illustrated) enclosing the upper active patterns 14, gate electrodes 18 formed on the gate insulation layer patterns, first impurity regions 20 formed at surface portions of the lower active patterns 12 and second impurity regions 22 formed at surface portions of the upper active patterns 14.
The gate electrodes 18 are formed between the first impurity regions 20 and the second impurity regions 22 so that the perpendicular channels are formed on the upper active patterns 14.
The first impurity regions 20 extend along a predetermined direction substantially the same as an extending direction of each of the lower active patterns 12. The first impurity regions 20 serve as bit lines. The bit lines are referred to as buried bit lines because the bit lines are formed under the transistor.
Electrical resistances of the first impurity regions 20 may increase because areas of the first impurity regions 20 adjacent to the upper active patterns 14 decrease. Namely, electrical resistances of the bit lines may increase so that the electrical reliability of the semiconductor device including the bit lines may be considerably reduced.